Precipitates and the like in metal materials have significant influences on, for example, material properties such as mechanical properties and electromagnetic properties depending on the morphology, size, and distribution thereof. In recent years, as techniques for enhancing steel properties using precipitates and the like have been greatly developed particularly in the field of iron and steel, the control of such precipitates and the like in manufacturing steps has become important.
In general, precipitates and the like contained in steel materials are grouped into those which increase properties, those which reduce properties, and those which do not contribute to properties. It is important to stably form precipitates and the like having a certain size and composition to produce steel materials with desired properties. In, for example, precipitation-hardened high-tensile strength steel sheets, attempts have been made to form fine precipitates and the like to allow the steel sheets to have increased tensile strength and nanometer- to sub-micrometer-sized extremely fine precipitates and the like are recently controlled. Therefore, there are strong demands for methods capable of quantitatively analyzing elements contained in nanometer- to sub-micrometer-sized precipitates by size.
The Iron and Steel Institute of Japan, “Handbook of Iron and Steel 4th edition (CD-RM),” Vol. 4, Section 2, 3.5 discloses techniques such as acidolysis, halolysis, and electrolysis, for quantifying precipitates and the like in steel materials. The Iron and Steel Institute of Japan, “Handbook of Iron and Steel 4th edition (CD-RM),” Vol. 4, Section 2, 3.5 describes that an electrolytic method performed in accordance with a procedure shown in FIG. 1 is particularly excellent. In the electrolytic method, an iron matrix is dissolved in an electrolytic solution and a filter is used as a solid-liquid separator for collecting precipitates and the like dispersed in the electrolytic solution. All the precipitates and the like are collected by the combination of the aggregation of relatively small precipitates and the like with the clogging of pores of the filter by relatively large precipitates and the like. That is, the aggregated relatively small precipitates and the like are deposited on the filter because of the clogging of the filter pores by the relatively large precipitates and the like and the precipitates and the like deposited on the filter fulfill a cake filtration function (a filtration mechanism in which the deposited precipitates and the like serve as a filter), whereby all the precipitates and the like are collected. Therefore, the whole of the precipitates and the like can be analyzed. However, no information on the size of the precipitates and the like can be obtained.
Several techniques for quantifying precipitates and the like by size have been proposed on the basis of the techniques disclosed in The Iron and Steel Institute of Japan, “Handbook of Iron and Steel 4th edition (CD-RM),” Vol. 4, Section 2, 3.5. However, these techniques are central to breaking the aggregation of precipitates and the like and/or preventing the formation of cake layers (sedimentary layers of precipitates and the like on filters). For example, Japanese Examined Patent Application Publication No. 53-37595 discloses a technique in which non-metallic inclusions in steel materials are chemically separated in a liquid, ultrasonic waves are effectively applied to the liquid during filtration using a metal filter such that the aggregation of the non-metallic inclusions is broken and the formation of cake layers is prevented, and the non-metallic inclusions are thereby fractionated by size. However, it is problematic to apply the technique disclosed in JP '595 to aggregates of nanometer- to sub-micrometer-sized extremely fine precipitates and the like although the technique is effective for coarse non-metallic inclusions of several micrometers or more. This is because since more fine particles exhibit stronger aggregation in liquids, it is difficult to break the aggregates of the nanometer- to sub-micrometer-sized extremely fine precipitates by applying ultrasonic waves to the aggregates and there is no metal filter having nanometer- to sub-micrometer-sized filter pores sufficient to exert the effect of ultrasonic waves. Japanese Unexamined Patent Application Publication No. 58-119383 discloses a technique in which precipitates and the like with a size of 1 μm or less are fractionated by ultrasonic vibration using an organic filter with a filter pore size of 1 μm or less. However, it is difficult for the technique disclosed in Japanese Unexamined Patent Application Publication No. 58-119383, as well as that disclosed in JP '595, to break aggregates of such fine precipitates and the like with a size of 1 μm or less by ultrasonic vibration. Unlike metal filters, organic filters insufficiently propagate or reflect ultrasonic waves because of material properties. Therefore, filter pores cannot be unclogged by ultrasonic vibration and cake layers are formed on the filters. Hence, precipitates cannot be fractionated in accordance with the filter pore size. The Japan Institute of Metals, “Materia Japan,” Vol. 45, No. 1, p. 52 (2006) discloses a technique in which precipitates and the like in a copper alloy are fractionated by size such that filtration is performed twice using filters having different filter pore sizes. However, the technique disclosed in The Japan Institute of Metals, “Materia Japan,” Vol. 45, No. 1, p. 52 (2006) is not effective in solving problems associated with the aggregation of precipitates and the like or the formation of cake layers. Hence, analysis by size cannot be accurately performed.
Conventional techniques have problems with the aggregation of precipitates and the like and formation of cake layers as described above and, therefore, are not capable of accurately and quantitatively analyzing nanometer- to sub-micrometer-sized precipitates and the like (particularly precipitates and the like with a size of 1 μm or less and more preferably 200 nm or less) by size.
It could therefore be helpful to provide a method for accurately and quantitatively analyzing nanometer-sized fine precipitates and the like contained in a metal material by fractionating the precipitates and the like by size.